Conventionally, NAND and NOR flash memories have been widely employed as nonvolatile semiconductor memory devices capable of electrical programming and erasing. In recent years, as tunnel oxide films has gotten thinner and thinner along with scaling of and increases in capacity of the memory cells of flash memories, there have been problems with shifts (read disturbs) in threshold due to the application of read voltages.
However, under actual use conditions, there might be applications where read operation is carried out during device operation; and since the frequency of reading is high especially in the code storage region in which the code of a program or the like is stored, restrictions on the number of times reading happens are unacceptable. Further, some memory controllers are mounted with a read disturb error self-restoring function (automatic recovery function). This function is fulfilled by placing a high-performance controller outside of a memory chip and saving and rewiring data on a block where read disturbs occurred and have been accumulated, and as such, requires an external buffer memory for use in data save and processing of ECC error correction information. No one unit chip has fulfilled such a function. Further, a technique for transferring data to another region cannot be adopted in a code storage memory, such as a NOR flash memory, in which addresses and data correspond to each other.
The applicant of the present invention proposes a B4-Flash memory using PMOS transistors as its memory cells unlike the conventional NAND and NOR flash (Patent Document 1). In the B4-Flash memory, the memory cells are PMOS, and therefore have negative thresholds, which results in a configuration of thresholds in which the absolute value of a threshold in erased state is large and the absolute value of a threshold in programmed state is small. In the B4-Flash, a read disturb causes a threshold in programmed state to shift toward erased state.
In the conventional NAND and NOR flash, on the other hand, a read disturb causes a shift in the opposite way to the B4-Flash, i.e., causes a threshold in erased state to shift toward programmed state.
It is an object of the present invention to recover a threshold shift on a cell of the B4-Flash whose threshold has shifted due to a read disturb.